Spectroscopic and computational studies of matrix-isolated iso-CXBr3 (X = F, Cl,Br): Structure, properties,
and photochemistry of substituted iso-tribromomethanes
OSU 67th International Symposium on Molecular Spectroscopy
Columbus, June 18th -22th , 2012
Aimable Kalume a, Lisa George a , Scott A. Reid a, Brian J. Esselman b, Robert J.McMahon b
a: Marquette Universityb: University of Wisconsin-Madison
2OSU 67th International Symposium on Molecular Spectroscopy
Introduction
1: Wayne, R. P. Chemistry of Atmosphere, 3rd ed.,Oxford University Press: Oxford, U.K., 20002:Molina et al., Nature, 1974, 249, 810-8123: Moore R. M. et al, J. Geophys. Res.-Oceans, 1996, 101, N0C9, 20899-20908 4: Quack, B. et al., Global Biogeochem. Cycles 2003, 17, Art. No. 1023
• Several polyhalomethanes have been detected in natural environment and are an important source of halogen atoms; involved in the “ozone depletion” phenomenon. 1-3
• Bromoform and its derivatives are major producers of Br atom in the troposphere and mid-latitude lower stratosphere. 1,4
• In condensed phase, the photolysis of bromoform led to facile production of the iso-bromoform
• In comparison with bromoform, less is known about the substituted tribromomethanes iso-CXBr3 (X = F, Cl, Br, I)
3OSU 67th International Symposium on Molecular Spectroscopy
Background: our previous work on iso-bromoform 1
1: L. George et al., J. Chem. Phys. 135, 124503 (2011)
0
50
100
150
200
250
300
350
400
Rela
tive e
nerg
ie in
kJ/m
ol
CHBr3
CHBr3(TS)251
204iso-CHBr3
CHBr + Br2
363
CHBr2 + Br267
252CBr2 + HBr
• Other iso-polyhalomethanes species that have been studied in our lab: iso-CF2I-I, iso-CF2Br-Br, iso-CH2Cl-I
Rela
tive
ener
gy in
kJ/
mol
4OSU 67th International Symposium on Molecular Spectroscopy
Optimized structures of iso-CXBr3
• Structures optimized at the MP2/aug-cc-pVTZ
• General consistency with M06/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ
• Some discrepancies between MP2 and DFT
5OSU 67th International Symposium on Molecular Spectroscopy
iso-CFBr3 : experimental results (IR and UV/vis)
• CFBr3:Ar[Ne] (1:500)
• Photolysis at 230nm followed by annealing to 30K [9K]
• In comparison with computational predictions (B3LYP/aug-cc-pVTZ and TDM06/aug-cc-pVTZ)
421
852
1161
288
432
594
6OSU 67th International Symposium on Molecular Spectroscopy
Calculated stationary points on the CFBr3 PES.
0
100
200
300
400
Rel
ativ
e en
ergie
in k
J/m
ol
CFBr2 + Br255
CFBr3 (TS)261
202iso-CFBr3
250CFBr + Br2
405CBr2 + FBr
CFBr3
Rela
tive
ener
gy in
kJ/
mol
7OSU 67th International Symposium on Molecular Spectroscopy
iso-CClBr3 : experimental results (IR)
• CClBr3:Ar (1:500)
• Photolysis at 220nm (544kJ/mol) followed by annealing to 30K
• In comparison with computational predictions (M06/aug-cc-pVTZ)
• Bond cleavage threshold: C-Cl: 293 kJ/mol C-Br: 232 kJ/mol
878
77980
981
6
8OSU 67th International Symposium on Molecular Spectroscopy
iso-CClBr3 : experimental results (UV/vis)
• CClBr3:Ar (1:500)
• Photolysis at 220nm (544kJ/mol) followed by annealing to 30K
• In comparison with computational predictions (TD-M06/aug-cc-pVTZ)
9
0
50
100
150
200
250
300
Rela
tive E
nerg
y in k
J/m
ol
CClBr3
(0)
iso-TS(229)
Calculated stationary points on the CClBr3 PES.
iso-ClBrCBr-Br(180)
iso-Br2CCl-Br(199)
iso-Br2CBr-Cl(200)
CClBr2 + Br(246)
CBr3 + Cl(277) CClBr + Br2
(272)
OSU 67th International Symposium on Molecular Spectroscopy
• CCSD(T)//MP2/aug-cc-pVTZ
• The iso-TS depicted in this figure is the transition state to the most stable iso-CClBr3 isomer.
10OSU 67th International Symposium on Molecular Spectroscopy
iso-CBr4 : experimental results (IR)
• CBr4:Ar (1:500)
• Photolysis at 266nm followed by annealing to 30K
• In comparison with computational predictions (M06/aug-cc-pVTZ)
842
781
11OSU 67th International Symposium on Molecular Spectroscopy
iso-CBr4 : experimental results (UV/Vis)
• CBr4:Ar (1:500)
• Photolysis at 266nm followed by annealing to 30K
• In comparison with computational predictions (TD-M06/aug-cc-pVTZ)
645
458
337
12OSU 67th International Symposium on Molecular Spectroscopy
Photoisomerization of iso-CXBr3
• CBr4:Ar (1:500)
• Irradiation into the strongest S0 →S3 band at 440nm
• The photolysis of the iso-CXBr3 results in the reformation of the parent isomer
• The exact mechanism has not been determined yet
13OSU 67th International Symposium on Molecular Spectroscopy
Calculated stationary points on the CBr4 PES.
• text
14OSU 67th International Symposium on Molecular Spectroscopy
Calculated resonance structure weights along IRC path
• First, calculate the IRC for the isomerization path
• Second, analysis of each IRC point using Natural Resonance Theory
• The TS near point 40 represent the cross-over from covalent to ion-pair binding.
15OSU 67th International Symposium on Molecular Spectroscopy
Conclusions
We have examined the structure, photochemistry and isomerization pathways of substituted iso-tribromomethanes
The electronic spectra were recorded for the first time
Stationary points on the PES were characterized computationally
The iso-species are relatively well bound with respect to the dissociation threshold
There is always a TS connecting both isomers and this was confirmed by IRC calculations
In condensed phases, the ion-pair structure is further stabilized with respect to the dissociation threshold
The photolysis of the iso-species results in the reformation of the parent CXBr3 isomer
16OSU 67th International Symposium on Molecular Spectroscopy
Acknowledgments
Dr. Scott A. ReidDr. Lisa GeorgeDr. Rajendra RathoreKhushabu Thakur
Dr. Robert J. McMahonBrian J. Esselman
National Science FoundationPetroleum Research Fund of the ACS
17OSU 67th International Symposium on Molecular Spectroscopy
Calculated resonance structure weights along IRC path
Cl
CBr Br
Br
Cl
CBr Br
Br
Cl
CBr
Br
Br
Cl
CBr Br
Br
18OSU 67th International Symposium on Molecular Spectroscopy
Calculated resonance structure weights along IRC path
Br
CBr Br
Br
Br
CBr Br
Br
Br
CBr Br
Br
Br
CBr Br
Br
Br
CBr Br
Br
19OSU 67th International Symposium on Molecular Spectroscopy
Calculated stationary points on the CClBr3 PES.
• textThe iso-TS depicted in this figure is the transition state to the most stable iso-CClBr3 isomer.